Official Blizzard Quote:

Merry New Year! The celebration begins at 6:00 a.m. Realm-Time December 31 with fireworks in the skies above the cities, and repeats every hour on the hour. Feel free to sample some of the exotic brews and libations carted in from around the world, and dance in celebration as a new year dawns. If you're on a PvP realm you're cautioned to steer clear of Booty Bay (or potentially visit with haste) as the guards there appear to have had too much fun already and aren't likely to be enforcing the town rules until morning.